In storage of information in magnetic recording media, reliability of storage of data is of paramount importance. The trend in modern data processing is towards ever smaller units of the data processing system. In the case of magnetic recording media, this means that the magnetic information is stored in a smaller area of the medium than before. The problem that arises is that it becomes more critical that the medium and the magnetic recording head reading the medium be capable of being used accurately. The head must be moved to exactly the correct position where the information is supposed to be stored on the medium. If the medium remains mechanically stable then the servo control system and the drive which move the magnetic recording head into position to read the magnetic recording medium can position itself based upon the known location of the data on the substrate. However, currently utilized magnetic recording media are magnetostrictive which means that as information is stored in the medium, the magnetostrictive forces generated can alter the stored information. Accordingly, we have found that it is highly desirable for the magnetic recording medium to be composed of materials which have as little magnetostriction as possible. Preferably, the magnetic recording medium should have zero magnetostriction.
U.S. Pat. No. 3,755,796 of Griest for "Cobalt-Platinum Group Alloys whose Anistropy is Greater than their Demagnetizable Field for use as Cylindrical Memory Elements", describes epitaxially sputtering a cobalt containing target in an inert gas to produce single crystal hexagonal cobalt (Co) films to produce a cylindrical domain structure. It is suggested in that connection that the target include precious metals such as ruthenium, rhenium, etc. In Table I, it is started that from 5-12 at. % of platinum can be added to Co to depress the demagnetizing field of Co while retaining a high anisotropy field by stabilizing the hexagonal phase structure of Co to higher temperatures. In claim 6 it is stated that 5-25 at. % of Pt can be added to Co. Ruthenium and rhenium and rhodium could be included respectively up to 35, 25 and 20 at. % in the alloy. Indium and osmium could be added up to 40 atomic percent. The patent is directed to a material for a cylindrical domain memory. The magnetic parameters of the material are not cited. No suggestion of a low magnetostriction, or high coercivity film are made. A polycrystalline film is not suggested nor is a face centered cubic structure.
U.S. Pat. No. 4,202,932 of Chen et al for "Magnetic Recording Medium", describes a magnetic recording medium composed of an alloy of cobalt with rhenium, ruthenium or osmium. The material has a coercivity of up to 800 Oe. The saturation induction can reach 5000 gauss. The film can be made by sputtering, among other thin film deposition techniques. The preferred atomic percentages of the Re, Ru or Os in the admixture is from 5 to 15 at. % at col. 4, line 64 it is stated that the platinum (group VIII) metal is from 2-25 at. %. No mention of magnetostriction is made.
Shirathata, U.S. Pat. No. 3,929,604 for "Method for Producing Magnetic Recording Medium", describes producing such films of Co-Pt among a large number of magnetic alloys by means of ionic plating. Coercivities of the alloys described in detail (other than Co-Pt) are under 400 Oe.
U.S. Pat. No. 3,625,849 of Rogalla, describes "Manufacture of Magnetic Medium" with high coercivity and low magnetostriction manufactured by sputtering followed by heating and annealing at above 600.degree. C. Cobalt is 25-50% of the Co-Cu alloy by weight. Other group VIII metals suggested are Fe and Ni. The other Group IB metal suggested is Au. Alloys suggested are Co-Au, Fe-Au, Fe-Cu and Ni-Au. No suggestion is made of use of Co-Pt alloys.
In U.S. patent application Ser. No. 956,296 of Michaelsen et al "Corrosion Resistant Magnetic Recording Media" teaches the use of Fe-Co-Cr compositions for magnetic recording media. However, we have found that the amount of Fe in the composition taught by Michaelsen et al produces a substantial value of magnetostriction since the amount of Fe varies on the phase diagram in FIG. 2 from 45 at. % up to 100 atomic percent and more importantly, the maximum amount of Co is 55 atomic percent which ranges down to 0% of Co. In accordance with this invention on the other hand, the minimum quantity of Co is 64 at. % and the amount of Co ranges up to 78 at. %. In either case, the amount of chromium (Cr) in the material is about the same. The Cr is present to provide corrosion resistance. It should be emphasized at this point that the objective of the Michaelsen et al patent was to provide a new magnetic recording media which is corrosion resistant. No mention is made there of the problem caused by magnetostriction, because Michaelsen et al were emphasizing the earlier critical problem of corrosion of magnetic recording media which is even more serious than the problem of magnetostriction. At one point in FIG. 3A Ref. 1.3 lists a composition of Fe 19 atomic percent, Co 67 atomic percent and Cr 14 atomic percent among many other materials "deposited by sputtering process". Nothing in the reference suggests that there is a possible advantage to the use of such a composition. It is merely mentioned in a large amount of data which is related to materials which are not useful.
In summary, Michaelsen et al Ser. No. 956,296 describes use of a magnetic medium of 0-55 atomic percent Co, 8-22 atomic percent Cr, with the remainder Fe (23-77 at. %).
The percentage of Cr in the alloy is about the same but the percentages of Fe and Co are quite different with the optimum value of Fe percentage about 13 at. % for this and the low end of the reference 23 at. % Fe which is greater than the highest permissible value of Fe of 21 at. % in this disclosure. The alloys are essentially nearest neighbors in that the only change is that the Fe-Co ratio has been reduced. However, in FIG. 3A, reference 1.3, the composition was Fe 19 atomic percent, Co 67 atomic percent, and Cr 14 atomic percent which provided H.sub.c of 110 Oe; M.sub.s 1050 emu/g; M.sub.r 670 emu/g and S0.63 and corrosion of 40.
U.S. Pat. No. 3,614,893 of Nesbitt et al for a "Splat Cooled Fe-Co-Cr Alloys and Devices Using Same" used as the core of an inductive thermometer because of paramagnetic properties with Fe 15-55% by weight, Co 45-65% by weight, and Cr or V of 10-20% by weight for sensing a temperature dependent change in magnetization of the mass. No statement relative to magnetostriction is made. It is seen however, that the weight percentage of 65% at the top for Co for Fe 23%, Cr 12%, Co 65% converts to Fe 23.6 at. %, Cr 13.2 at. % and Co 62.3 at. % which fails to overlap the bottom of 64 at. % for Co for the present invention. In their Example III, the alloy was 12 weight % Cr, 52 weight %, Co and 36 weight % Fe which converts to Cr 13.1 at. %, Co 50.3 at. %, and Fe 36.7 at. %.
An article by Klokholm and Tan entitled "Sputtering FeCoCr Thin Film Magnetic Media", IBM Technical Disclosure Bulletin 21, No. 10 4241 (March 1979) calls for a high Cr content for corrosion resistance and later quantifies the Cr content as up to 10 at. %. Copending U.S. application Ser. No. 221,867 of Aboaf et al, describes a "Zero Magnetostriction FE-CO-CR Magnetic Alloy". The alloy is (Fe.sub.y Co.sub.1-y).sub.1-x Cr.sub.x where y (Fe) is preferably 15-23 atomic percent of the Fe-Co part of the alloy. The value of x (Cr) is 7-20 atomic percent of the alloy and the remainder 1-x (Fe-Co) is 83-92 atomic percent of the alloy. The maximum ranges of the composition of the alloy are about as follows:
Fe-8-24 atomic percent PA1 Co-56-83 atomic percent PA1 Cr-7-20 atomic percent
Much work has been done in the ranges of high concentrations of Fe, above 50 at. % and low concentrations of Cr about 1 at. % or less.
None of the prior art suggests the use of the particular range of Co-Pt alloys of this invention for a low magnetostriction alloy for use as a magnetic recording medium.
While the prior art Griest patent suggests the broad range of Co-Pt materials deposited by sputtering, he does not suggest a high coercivity, polycrystalline film or a low or zero magnetostriction film, and his range of at. % of Pt in the alloy is too broad to produce the desired results reliably. Furthermore, he does not teach a polycrystalline thin film with a combination of the fcc phase and the hexagonal phase which yields the high coercivity we have discovered. Such high coercivity is essential to the applications we envision in magnetic recording disks and hard biasing of thin film magnetic magnetoresistive recording heads.
The chief interest in bulk alloys in the Co.sub.1-y Pt.sub.y system has centered heretofore on alloys in the region of equiatomic composition. Such alloys when cooled from above 1000.degree. C. have a disordered face centered cubic crystal structure; upon annealing at 600.degree. C. the structure becomes face centered tetragonal. The very high coercivity (thousands of oersteds) obtained for these alloys is the result of the cooling rate from the disordering treatment, the aging, and to some extent it results from the variation in the platinum content around the stoichiometric value. In the region of equiatomic composition, we have been able to increase the coercivity of films from 40 oersteds (as deposited films) to 1000 oersteds after an annealing treatment at 600.degree. C.